(1) Field of the Invention
The present invention relates to electrochemical systems and more specifically to a novel semi-fuel cell design based on a seawater electrolyte and liquid catholyte combination that uses an ion exchange membrane to isolate the seawater electrolyte and liquid catholyte combination from the seawater anolyte solution.
(2) Description of the Prior Art
Primary batteries employing aqueous electrolytes have been under investigation for several years leading to the development of semi-fuel cells, a hybrid of fuel cells and batteries that combine the refillable cathode or catholyte oxidizer of fuel cells with the consumable anode fuel of batteries. The metal anode and liquid catholyte are consumed to produce energy. Semi-fuel cells are currently being considered as electrochemical energy sources for unmanned undersea vehicles due to the availability of seawater to act as an electrolyte in combination with the liquid catholyte or alone as the anolyte solution. The semi-fuel cell anode is often made of aluminum, magnesium or lithium due to the high faradaic capacity, low atomic weight and high standard potential of these metals. The catholyte is usually a strong oxidizer such as hydrogen peroxide in solution with the seawater electrolyte.
Seawater semi-fuel cells, also known as solution phase semi-fuel cells because the catholyte is in solution with the seawater, are an ideal electrochemical energy source for undersea vehicles. The use of seawater from the undersea vehicle's surroundings minimizes the volume and weight of reactants that need to be carried in the vehicles. This provides an important weight savings to the vehicle. Seawater semi-fuel cells have a high faradaic capacity, and have a high energy density at low current densities while being relatively inexpensive, environmentally friendly, capable of a long shelf life, and not prone to spontaneous chemical or electrochemical discharge.
In order to meet the high energy density requirements of underwater vehicles semi-fuel cells currently being developed are used in stack or multi-stack configurations. The use of bipolar electrodes having an anode on one side and a catalyzed cathode on the other is beneficial in minimizing cell stack size and weight.
In prior art semi-fuel cells, each cell is hydraulically fed in parallel with a seawater and/or sodium hydroxide (NaOH) aqueous electrolyte. The catholyte is carried separately and injected directly into the seawater and/or seawater/sodium hydroxide mixture upstream of the stack inlet at the required concentration, determined by the system power load. Electrochemical reduction of the catholyte, occurs on the electrocatalyst surface of the cathode current collector, receiving electrons from the anode oxidation reaction.
The electrochemical reactions for an aluminum hydrogen peroxide semi-fuel cell are given below:Cathode: HO2−+H2O+2e−->3OH− E°=0.88 VAnode: Al+4OH−->AlO2−+2H2O+3e− E°=−2.33 VCell Reaction: 2Al+3HO2−->2AlO2−+OH−+H2O Ecell=3.21 V
In addition to the primary electrochemical reaction, the following undesired parasitic reactions can also take place:Corrosion: 2Al+2H2O+2OH−->2AlO2−+3H2↑Direct Reaction: 2Al+3H2O2+2OH−->2AlO2−+4H2ODecomposition: 2H2O2->2H2O+O2↑
The electrochemical reactions for a magnesium hydrogen peroxide semi-fuel cell are given below:Anode: Mg->Mg2++2e− E°=−2.37 vCathode: H2O2+2H++2e−->2H2O E°=1.77 vCell Reaction: Mg+H2O2+2H+->Mg2++2H2O Ecell=4.14 v
In addition to the primary electrochemical reaction, the following undesired parasitic reactions could also take place:Decomposition: 2H2O2->2H2O+O2(g) Direct Reaction: Mg+H2O2+2H+->Mg2++2H2OCorrosion: Mg+2H2O->Mg2++2OH−+H2(g) 
The electrochemical reactions for the lithium-hydrogen peroxide semi-fuel cell are given below:Anode: Li->Li++e− E°=−3.04 vCathode: H2O2+2H++2e−->2H2O E°=1.77 vCell Reaction: 2Li+H2O2+2H+->2Li++2H2O Ecell=4.81 v
In addition to the primary electrochemical reactions, the following undesired parasitic reactions could also take place:Decomposition: 2H2O2->2H2O+O2 Direct Reaction: Li+H2O2+2H+->2Li++2H2OCorrosion: 2Li+2H2O->2Li++2OH−+H2(g) 
Of the parasitic reactions listed above, the direct reactions are the most detrimental to the operation of the semi-fuel cell since both the metal anode, either magnesium, aluminum or lithium and the the hydrogen peroxide catholoyte are consumed in a single step. A direct reaction occurs when the catholyte, in this case H2O2, is allowed to come into direct physical contact with the metal anode, resulting in a chemical reaction which does not produce electron transfer and only consumes active energetic materials, thus reducing the overall energy yield of the semi-fuel cell. In most cases this parasitic reaction will consume over 50% of the available energetic materials.
Whereas magnesium, lithium or aluminum corrosion can be suppressed by pH adjustment and hydrogen peroxide decomposition minimized by careful temperature control, in order to minimize or completely prevent the parasitic direct reaction, the metal anode side of the bipolar electrode must be physically isolated from the liquid catholyte.
What is needed is a semi-fuel cell that enables the separation of the metal anode from the catholyte while maintaining necessary ion transfer to affect the necessary electrochemical balance for the reaction to take place. This is accomplished through a new semi-fuel cell design that incorporates an ion exchange membrane to allow a separated flow of anolyte and catholyte in the semi-fuel cell thereby isolating the metal anode of the bipolar electrode from the catholyte.